1. Field
The following relates recording media with enhanced areal density, and more particularly to methods, systems and components that allow for reduced magnetic spacing through reduction of head media spacing, head keeper spacing, or head to soft underlayer spacing.
2. Related Art
Magnetic media are used in a variety of applications, predominantly in the computer and data storage industries, for example, in devices such as hard disk drives and other recording devices. Areal density, also called bit density, refers to the amount of data that can be packed onto a storage medium per unit area. Areal densities are generally measured in gigabits per square inch. Current magnetic and optical disks have areal densities of several gigabits per square inch. Efforts are being made to increase the areal recording densities of media to greater than 200 gigabits per square inch. In this regard, perpendicular recording media structures have been found to be superior to the more conventional longitudinal media in achieving high areal densities. Magnetic recording media are generally classified as “longitudinal” or “perpendicular” depending upon the orientation of the magnetic domains of the grains of magnetic material; the magnetic recording media of the present invention can include perpendicular recording media, longitudinal recording media, discrete track recording media, bit patterned media, or heat assisted magnetic recording (HAMR) media.
In perpendicular magnetic recording media (recording media with a perpendicular anisotropy in the magnetic layer), remanent magnetization is formed in a direction perpendicular to the surface of the magnetic medium, and the recorded bits are stored in a perpendicular, or out-of-plane, orientation in the recording layer.
In conventional thin-film type magnetic media, a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer. Typically, recording media are fabricated with polycrystalline CoCr or CoPt-oxide containing films. Co-rich areas in the polycrystalline film are ferromagnetic while Cr or oxide rich areas in the film are non-magnetic. Magnetic interaction between adjacent ferromagnetic domains is attenuated by nonmagnetic areas in between.
High linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with perpendicular magnetic media. The write transducer or head can comprise a main (writing) pole as well as auxiliary poles and creates a highly concentrated magnetic field which alternates the media magnetization direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
A typical perpendicular recording system utilizes a magnetic medium with a relatively thick (as compared with the magnetic recording layer) “soft” magnetic underlayer (SUL), a relatively thin “hard” perpendicular magnetic recording layer, and a single-pole head. Magnetic “softness” refers to a magnetic material having a relatively low coercivity of about 2-150 oerstads (Oe) or preferably of about 1 kOe, such as of a NiFe alloy (Permalloy) or a material that is easily magnetized and demagnetized. The magnetically “hard” recording layer has a relatively high coercivity of several kOe, typically about 2-10 kOe or preferably about 3-8 kOe, and comprises, for example, a cobalt-based alloy (e.g., a Co-Cr alloy such as CoCrPtB or a material that neither magnetizes nor demagnetizes easily) having perpendicular anisotropy. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer. The system further preferably comprises a non-magnetic substrate, at least one non-magnetic interlayer, and an optional adhesion layer. The relatively thin interlayer comprised of one or more layers of non-magnetic materials, is preferably positioned below the at least one magnetically hard recording layer, and serves to prevent magnetic interaction between the soft underlayer and the magnetically hard recording layer and promote desired microstructural and magnetic properties of the hard recording layer. See US Publication No. 20070287031; U.S. Pat. Nos. 6,914,749; 7,201,977. The interlayer may comprise multiple layers forming an interlayer stack, with at least one of these layers preferably including an hcp (hexagonally close packed) material adjacent to the magnetically hard perpendicular recording layer.
Magnetic flux φ emanates from the main writing pole of the magnetic head, enters and passes through the at least one vertically oriented, magnetically hard recording layer in the region below the main pole, enters and travels within the SUL for a distance, and then exits therefrom and passes through the at least one perpendicular hard magnetic recording layer in the region below an auxiliary pole of the transducer head.
Granular perpendicular magnetic recording media is being developed for its capability of further extending the areal density of stored data, as compared to conventional perpendicular media, which is limited by the existence of strong lateral exchange coupling between magnetic grains. A granular (meaning that the in-plane grains are discontinuous in nature) perpendicular recording medium comprises a granular perpendicular magnetic layer having magnetic columnar grains separated by grain boundaries comprising voids, oxides, nitrides, non-magnetic materials, or combinations thereof. The grain boundaries, having a thickness of about 2 to about 20 angstroms (Å), provide a substantial reduction in the magnetic interaction between the magnetic grains. In contrast to conventional perpendicular media, wherein the perpendicular magnetic layer is typically sputtered at low pressures and high temperatures in the presence of an inert gas, such as argon (Ar), deposition of the granular perpendicular magnetic layer is conducted at relatively high pressures and low temperatures and utilizes a reactive sputtering technique wherein oxygen (O2) and/or nitrogen (N2) are introduced in a gas mixture of, for example, Ar and O2, Ar and N2, or Ar and O2 and N2. Alternatively, oxygen or nitrogen may be introduced by utilizing a sputter target comprising oxides and/or nitrides, which is sputtered in the presence of an inert gas (e.g., Ar), or, optionally, may be sputtered in the presence of a sputtering gas comprising O2 and/or N2 with or without the presence of an inert gas. The introduction of O2 and/or N2 provides oxides and/or nitrides that migrate into the grain boundaries can provide for a granular perpendicular structure having a reduced lateral exchange coupling between grains. See US Publication No. 20060269797. The introduction of such grain boundaries can increase the areal density of recording/storing media.
The interposition of the various layers within a medium described herein forms a stacked structure. The layer stack of the medium contains grain boundaries within the polycrystalline layers. Since a magnetically hard main recording layer is preferably epitaxially formed on the interlayer, the grains of each polycrystalline layer are of substantially the same width (as measured in a horizontal direction) and in vertical registry (i.e., vertically “correlated” or aligned). Completing the layer stack is a protective overcoat layer, such as of a diamond-like carbon (DLC), formed over the hard magnetic layer, and a lubricant topcoat layer, such as of a perfluoropolyether material, formed over the protective overcoat layer. The perpendicular recording medium may also comprise a seed layer which is preferably adjacent to the magnetically soft underlayer (SUL) and preferably comprises at least one of an amorphous material and a face-centered-cubic lattice structure (fcc) material. The term “amorphous” means that such a material exhibits no peak in an X-ray diffraction pattern as compared to background noise. Amorphous layers according to this invention may encompass nanocrystallites in amorphous phase or any other form of a material so long the material exhibits no peak in an X-ray diffraction pattern as compared to background noise. A seed layer seeds the nucleation of a particular crystallographic texture of the underlayer. Conventionally, a seed layer is the first deposited layer on the non-magnetic substrate. The role of this layer is to texture or align the crystallographic orientation of the subsequent Cr-containing underlayer. The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of argon.
Vertically stacked magnetic layers comprising a so-called “granular” recording layer (wherein the magnetic grains are only weakly exchange coupled laterally) and a continuous layer (wherein the magnetic grains are strongly exchange coupled laterally) are ferromagnetically coupled together in certain recording medium configurations. In such media, the entire continuous magnetic layer may couple with each grain in the granular magnetic layer (forming a vertically exchange coupled composite—“ECC”). See U.S. Pat. No. 7,201,977.
Very fine-grained magnetic recording media may possess thermal instability. One solution is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. This can be achieved by providing a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (“AFC”) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability; the coupling strength between the ferromagnetic layer pairs being a key parameter in determining the increase in stability. A continuous ferromagnetic layer has a lower coercivity than that of a discontinuous ferromagnetic layer; a non-magnetic spacer layer provides magnetic or anti-ferromagnetic coupling between the continuous ferromagnetic layer and the discontinuous ferromagnetic layer depending upon its thickness. Preferably, the magnetic grains of the upper and lower magnetic layers are grown in vertical alignment and are equal or about equal in size; otherwise, the areas written in each of the pair of ferromagnetic layers may not coincide. U.S. Pat. No. 6,777,112.
The substrate is typically disk-shaped and may comprise glass, ceramic, glass-ceramic, NiP/aluminum, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials non-magnetic materials, or a combination or a laminate thereof. See U.S. Pat. No. 7,060,376. A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al—Mg) alloy. Such Al—Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture. The optional adhesion layer, if present on the substrate surface, typically comprises a less than about 200 angstroms (Å) thick layer of a metal or a metal alloy material such as Ti, a Ti-based alloy, Ta, a Ta-based alloy, Cr, or a Cr-based alloy.
The relatively thick soft magnetic underlayer is typically comprised of an about 50 to about 300 nm thick layer of a soft magnetic material such as Ni, Co, Fe, an Fe-containing alloy such as NiFe (Permalloy), FeN, FeSiAl, FeSiAlN, a Co-containing alloy such as CoZr, CoZrCr, CoZrNb, or a Co—Fe-containing alloy such as CoFeZrNb, CoFe, FeCoB, and FeCoC. The relatively thin interlayer stack typically comprises an about 50 to about 300 Å thick layer or layers of non-magnetic material(s). The interlayer stack includes at least one interlayer of an hcp material, such as Ru, TiCr, Ru/CoCr 37Pt6, RuCr/CoCrPt, etc., adjacent the magnetically hard perpendicular recording layer. When present, a seed layer adjacent the magnetically soft underlayer (SUL) may typically include a less than about 100 Å thick layer of an fcc material, such as an alloy of Cu, Ag, Pt, or Au, or an amorphous or fine-grained material, such as Ta, TaW, CrTa, Ti, TiN, TiW, or TiCr. The at least one magnetically hard perpendicular recording layer is typically comprised of an about 10 to about 25 nm thick layer(s) of Co-based alloy(s) including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, W, Cr, Ru, Ti, Si, O, V, Nb, Ge, B, and Pd.
Of the conventional media types described above, longitudinal media are more developed than perpendicular media and have been utilized for several decades in the computer industry. During this interval, components and sub-systems, such as transducer heads, channels, and media, have been repeatedly optimized in order to operate efficiently within computer environments. However, it is a common current belief that longitudinal recording is reaching the end of its lifetime as an industry standard in computer applications owing to physical limits which effectively prevent further increases in areal recording density.
Perpendicular media, on the other hand, are expected to replace longitudinal media in computer-related recording applications and continue the movement toward ever-increasing areal recording densities far beyond the capability of longitudinal media. However, perpendicular media and recording technology is less well developed than all facets of longitudinal media and recording technology. Specifically, each individual component of perpendicular magnetic recording technology, including transducer heads, media, and recording channels, is less completely developed and optimized than the corresponding component of longitudinal recording technology. As a consequence, the gains observed with perpendicular media and systems vis-à-vis the prior art, i.e., longitudinal media and systems, are difficult to assess.
High density perpendicular recording media require careful control and balance of several magnetic properties including: high enough anisotropy to enable thermal stability and compatibility with a high gradient head; low enough switching field to enable writability by the head; lateral exchange coupling low enough to maintain small correlation length between magnetic grains or clusters and high enough to maintain a narrow switching field distribution (SFD); and grain-to-grain uniformity of magnetic properties sufficient to maintain thermal stability and minimize SFD.
As recording density continues to increase, it is necessary to make smaller grain structures to maintain the number of magnetic particles in a bit at a similar value. Smaller grain structures are more sensitive to non-uniformities such as anisotropy variations within grains, and also require higher anisotropy to maintain thermal stability, thus adversely affecting writability. Therefore, there is a need in the art for a media with improved writability and fewer defects for narrower SFD and improved uniformity of properties.
Current methods for enhancing areal density focus on manipulation and tweaking of elements within the magnetic recording layers of devices. However, head media spacing (HMS) (the distance between the magnetic writer head and the magnetic recording layer, excluding overcoats and lubricating coats on either head or recording layer) and head keeper spacing (HKS) (the gap between the writer air-bearing surface and the SUL) or head to SUL spacing are among the primary factors that limit areal density. As HMS/HKS is reduced, areal density increases. See FIG. 1. Every angstrom of reduction can be significant for increasing areal density. In the meantime, field gradient is also improved. However, it is very difficult to reduce HMS/HKS, and in recent years, the reduction of HMS/HKS has slowed down across the magnetic disk drive recording and storing industry.
One of the primary reasons for a limitation on areal density growth is that conventional scaling law cannot be maintained—in other words, the HMS/HKS cannot be scaled with reduction of head geometry. Further, a reduction of HMS/HKS physical spacing may not be desirable because of the advantages that an optimal amount of physical spacing may confer on the medium, such as functional perpendicular orientation, grain separation, and appropriate grain size in the magnetic recording layer.